diff --git a/app/controllers/zipdown_controller.rb b/app/controllers/zipdown_controller.rb
index 2be4e78bb..3612bf7a4 100644
--- a/app/controllers/zipdown_controller.rb
+++ b/app/controllers/zipdown_controller.rb
@@ -44,7 +44,7 @@ class ZipdownController < ApplicationController
       if homework != nil
         unless homework.attachments.empty?
           zipfile = zip_homework_by_user homework
-          send_file zipfile, :filename =>  (homework.user.user_extensions.nil? ? "" : homework.user.user_extensions.student_id) +
+          send_file zipfile, :filename =>  ((homework.user.user_extensions.nil? || homework.user.user_extensions.student_id.nil?) ? "" : homework.user.user_extensions.student_id) +
               "_" + (homework.user.lastname.nil? ? "" : homework.user.lastname) + (homework.user.firstname.nil? ? "" : homework.user.firstname) +
               "_" + homework.name + ".zip", :type => detect_content_type(zipfile) if(zipfile)
         else
@@ -80,7 +80,7 @@ class ZipdownController < ApplicationController
     homeattach.attachments.each do |attach|
       homeworks_attach_path << attach.diskfile#.to_s.slice((length+1)..-1)
     end
-    zipping("#{homeattach.user.lastname}#{homeattach.user.firstname}_#{(homework.user.user_extensions.nil? ? "" : homework.user.user_extensions.student_id)}_#{Time.now.to_i.to_s}.zip", homeworks_attach_path, OUTPUT_FOLDER, true)
+    zipping("#{homeattach.user.lastname}#{homeattach.user.firstname}_#{((homeattach.user.user_extensions.nil? || homeattach.user.user_extensions.student_id.nil?) ? "" : homeattach.user.user_extensions.student_id)}_#{Time.now.to_i.to_s}.zip", homeworks_attach_path, OUTPUT_FOLDER, true)
   end